1. Field of the Disclosure
This invention relates to an electrophotographic printer having an imaging device, and, more particularly, to a system and method for diagnosing potentially catastrophic mechanical failures occurring in the imaging device.
2. Description of the Related Art
In a color electrophotographic process, toner is transferred by electrostatic means to an intermediate transfer member (ITM) at each of four or more successive imaging stations, with each imaging station representing a different color plane. Toner is accumulated onto the ITM belt and then transferred onto a media sheet by reversing the electrostatic field. This transfer onto paper is not 100 percent efficient, and some small amount of toner is left on the ITM belt that needs to be removed prior to a subsequent image to be accumulated on the ITM belt.
Second transfer is where the image is transferred from the ITM belt to the sheet of media. At the second transfer, a second transfer roll and a backup roll form a transfer nip. The media of media enters into the second transfer nip between the second transfer roll and the ITM belt. Voltages are applied to the second transfer roll and the backup roll to develop an electric field therebetween. The electric field at second transfer is constructed in a way to pull the toner off of the ITM belt onto the sheet of media. Second transfer is not 100% efficient, so residual toner is left on the ITM belt. The residual toner after second transfer is typically cleaned before a new image is placed on the ITM belt. If the toner is not cleaned off of the belt, the residual toner could contaminate the next media sheet printed.
Following second transfer, a cleaning unit typically employs a blade to scrape residual toner from the ITM belt. The residual toner builds up and is moved away from the blade by an auger into a waste toner box. The waste toner box is periodically replaced by the customer.
Occasionally, a “toner dump” can occur in a printing system. Causes of a toner dump include a bias voltage problem at the developer roll of a developer unit; a seal failure in a toner bottle; a lower seal failure in a developer unit; the photoconductive drum failing; and the ITM belt cleaner unit failing due seal failure or the cleaner blade flipping. Without looking at the printed sheet of media, there is little way to detect the occurrence of a toner dump. The imaging device is blind to this failure mode until a catastrophic mechanical failure occurs that results in a service call. Unfortunately, not only are toner dumps not easily detectible but the causes of a toner dump can quickly result in mechanical failure.
If a toner dump occurs, toner around the auger in the cleaning unit bridges in such a way that toner does not drop down into the waste toner box. As a result of the toner blockage, the torque on the auger may increase as a result but does not cause the auger or belt to stall. The auger eventually builds up enough toner in the end of the cleaning unit to start to buckle the auger. Buckling the auger puts a downward force on the cleaner blade, which causes the blade to flip, the auger to seize, or the cleaning unit to leak. The ITM belt will be destroyed and must be replaced.
The volume in and around the auger of the cleaning unit could be increased to handle the rare occasion when toner is dumped onto the ITM belt, but this requires additional volume in the machine in a location that is typically already cramped for space. With additional volume comes additional cost.
Even if the volume around the auger were increased, the imaging device would still be susceptible to other downstream failure modes that are not as easily remedied. Some of the excess toner on the belt may be transferred to sheets of media. Excessive toner coverage on the media sheets caused by a toner dump may result in failure by the fusing assembly of the imaging device. Poor fuse grade may result in a fuser roll being undesirably wrapped by a sheet of media or toner contaminating media transport guides internal and external the fuser assembly. Both of these result in a fuser replacement or a service call. Additionally, widespread machine contamination resulting from a toner dump may result in replacement of components, assemblies and modules within the imaging device.
In addition, dual component development (DCD) based imaging devices are further complicated by conditions that can relatively quickly lead to mechanical failure within the imaging device. A DCD based imaging device is one in which a development mix contains a portion of polymeric resin based toner and a magnetic carrier. Typically, the magnetic carrier will have a polymeric coating constructed of a triboelectrically different resin than the toner. When the toner is mixed with the carrier, the toner will charge to one polarity, while the carrier coating will charge to the opposite polarity. At that point, the toner will adhere to the oppositely charged carrier bead.
The magnetic carrier is utilized for two purposes. One purpose is to allow for the charging of the toner particles to a range appropriate for toner development, and the second purpose is to allow for the usage of a magnetic roller to transport the toner into the development zone between the magnetic roller and the photoconductor.
The final charge on the toner particles is a function of multiple kinetic and material factors within the development mix. Some of these factors may be, but are not limited to, the material choices for the toner and the carrier coatings, the amount of mix energy utilized to combine the toner and the carrier, and the weight concentration of toner to the carrier in the developer mix. The weight concentration of toner to carrier is commonly referred to as the toner concentration (TC) of the developer mix. If the TC increases, then the available surface area of the carrier per toner particle decreases, reducing the charge of the toner. If the toner concentration increases, then the available surface area of the carrier per toner particle increases, increasing the charge of the toner.
To maintain substantially constant toner charge in a development system, the amount of toner printed out of the developer unit needs to be known and the same amount must be fed back into the development sump of the developer unit to maintain a level of TC within a specific operating range. Any transition to a TC outside of this range could cause irreparable damage to the development mix and potentially the entire imaging system. If the TC gets too high, the toner charge and the carrier charge will both get too low causing “fogging” or “dusting” of the final image with toner and the possibility of bead carry-out (BCO) which is the loss of carriers to the photoconductor. A very low TC could lead to premature aging of the carrier which can also result in low carrier charge leading to BCO. At high levels of BCO, the development sump of the developer unit will become depleted of carriers, significantly reducing the life of the development system. At extremely high levels of BCO, other imaging system components (photoconductors, cleaner blades, transfer belts, etc.) can become irreparably damaged.
Though both undesirable changes in TC should be noticed by the user of the imaging device, thereby providing the user with an opportunity to initiate remedial measures so as to avoid irreparable damage. However, an issue arises when a user sends a very long print job and does not visibly monitor the output. By the time the user of imaging device may notice that the image quality is not acceptable, irreparable damage may have already occurred to the development system and the imaging system as a whole.